1. Field
Aspects of the present invention relate to technologies of a particle radiation monitoring apparatus which acquires information on behaviors of particle beams irradiated by an accelerator etc., a recording medium to retain a particle radiation monitoring program, and a particle radiation monitoring method.
2. Description of the Related Art
A technology of radiotherapy for cancer in Japan has realized the tremendous improvements of achievements in medical treatments by reducing damages to peripheral normal healthy tissues to the greatest possible degree in a way that concentrates a radiation dose on a nidus under the slogan of “Remedy without Cutting Cancer”. The forefront radiotherapy for cancer has come to involve utilizing “particle therapy” which drastically reduces the radiation dose on the normal healthy tissues while irradiating an affected part with a large radiation dose and is on the verge of realizing the improvements of the achievements in medical treatments which could not be attained even by the high-level X-ray therapy. This is derived from “concentration of the radiation dose on the affected part” as a nature common to the particle beams such as proton beams and heavy particle beams. In addition, the heavy particle beams have superiority in terms of progressive migration of ions within a body and therefore exhibit a reduced deviation of the ions from a traveling direction. The heavy particle beams are also excellent in concentration of the radiation dose in a radius-vector direction with respect to the ion traveling direction, whereby a nidus of the affected part or a minute nidus part contiguous to important tissues can be precisely pinpoint-irradiated with the heavy particle beams. It is expected that an ion microsurgery technique is established as a next-generation therapy utilizing this feature of the heavy particle beams.
A pencil beam forming technology and a micro beam forming technology of converging the beams in a thin-and-narrow shape have already been developed as an accelerator or a beam technology for pinpoint-irradiating the nidus part precisely with the particle beams as described above. The particle radiation monitoring technology at the present cannot, however, monitor an internal arrival depth of the particle beams in real time, with the result that it is difficult to establish the ion microsurgery technique.
Further, a real-time monitoring technology for an energy impartation distribution (i.e., radiation dose distribution) of the nidus part undergoing the radiation therapy underway is not yet developed. Therefore, the medical treatment is performed based on a tremendous quantity of dose measurement data of a phantom through a physical or chemical technique as well as being based on a detailed therapeutic plan and empirically and clinically accumulated data. Under the present circumstances, priori confirmation of the energy impartation distribution is invariably made in the therapeutic plan and a QA (Quality Assurance) scheme, and, if there are no monitoring fluctuations of the beams during the therapeutic irradiation, the radiation therapy is carried out on the assumption that the energy distribution is reproduced and maintained. The realization of the real-time monitoring technology of the energy impartation distribution enables the medical treatment to be performed while confirming and demonstrating that the nidus part is certainly irradiated with the beams, and has an extremely large medical significance in terms of ensuring the reliability.
Further, if unpredictable filling and discharge of mucic occur in an internal cavity region during the irradiation of the particle therapy, the unpredictable filling and discharge considered by way of deviations from the therapeutic plan exert adverse influence as variations of the energy impartation distribution and the ion arrival depth. It is therefore of much benevolence to observe in real time the energy impartation distribution and the arrival depth during the irradiation of the particle beams.
In the present situation, the particle radiation monitoring techniques are exemplified (W. Enghardt et al., “The spatial distribution of positron-emitting nuclei generated by relativistic light ion beams in organic matter”, Phys. Med. Biol., 1992, Vol. 37, No 11, 2127-2131) (Katia Parodi et al., “PET imaging for treatment verification of ion therapy: Implementation and experience at GSI Darmstadt and MGH Boston”, Nucl. Instr. and Meth. A 591 (2008) 282-286). There is searched a technique of observing positron annihilation gamma rays defined as the gamma rays derived from positron emission nuclides (O-15, C-11, etc) generated due to the nuclear reaction between the ions and the internal atomic nucleuses and estimating an irradiation position (which will hereinafter be called a self-radiation method). As for the generation reaction of the positron emission nuclides, a majority of these nuclides are generated through not only primary reaction between the ions and the internal nuclides but also secondary reaction due to neutrons generated by the primary reaction and via complicated multiple reaction paths on the whole. Therefore, the Monte-Carlo simulation including the nuclear reaction is required for estimating a generation quantity and a generation place. Consequently, the reproduction of the energy impartation distribution from the positron distribution has a problem of requiring an analysis which traces the Monte-Carlo simulation including the complicated nuclear reaction.
Moreover, there is a time difference of several tens of seconds to several tens of minutes due to a decay period of the nuclides till the gamma rays (of, e.g., 511 keV) are emitted since the positron emission nuclides have been generated, so that a period ranging from several minutes to several tens of minutes is needed till the measurement of PET (Positron Emission Tomography) or CT (Computed Tomography) is finished since the end of the medical treatment. The positron emission nuclides migrate within the body by dint of a metabolic function inherent to a living body during this time difference. With this migration, such a problem arises that a deviation occurs between the generation position of the positron emission nuclides and the emission position of the gamma rays. This problem is called a washout effect due to the metabolism and is one of factors of complicating the prediction of the energy impartation distribution.
Measurement quantities in the particle radiation monitoring method are, e.g., the arrival depth of the particle beams and the energy impartation distribution. In the self-radiation method described above, a study for a technique of monitoring the arrival depth of the particle beams and the energy impartation distribution is advanced. In the self-radiation method, however, it is difficult to monitor in real time the arrival depth of the particle beams and the energy impartation distribution. The reason is that the positron emission nuclides generated in the self-radiation method has a problem of their being generated generally through atomic nucleus reaction exhibiting an extremely small reaction probability. Hence, a considerable period of time is expended for collecting the data required for presuming the arrival depth of the particle beams or the energy impartation distribution in the self-radiation method. To be specific, in the radiotherapy, the data required for presuming the arrival depth of the particle beams or the energy impartation distribution are obtained after finishing the medical treatment, and it is difficult to conduct the real-time monitoring during the medical treatment. Moreover, in the heavy particle beams expected for the forefront medical treatment, an irradiation quantity of the ions used for the medical treatment is approximately one tenth as small as that of proton beams, and it is further difficult to acquire the sufficient data.
Furthermore, as described above, the derivation of the quantities of the arrival depth of the particle beams and the energy impartation distribution involves the difficulty due to the intricacy of the generation reaction and the washout effect owing to the metabolism. As a technique of avoiding the washout effect, a method (which will hereinafter be called a nuclear de-excitation method) of observing the prompt gamma rays from excited atomic nucleuses generated by the atomic nucleus reaction between the particle beams and the internal atomic nucleuses is proposed (S. Kabuki et al., “Study on the Use of Electron-Tracking Compton Gamma-Ray Camera to Monitor the Therapeutic Proton Dose Distribution in Real Time”, 2009 IEEE Nuclear Science Symposium Conference Record, 2437-2440).
An occurrence count of the nuclear de-excitation gamma rays is also small because of their being via the atomic nucleus reaction, and the real-time monitoring is hard to perform. In the self-radiation method, two positron annihilation beams are simultaneously generated, and it is therefore feasible to employ an imaging apparatus based on the PET and the gamma-ray pair measurement similar to the PET. In the nuclear de-excitation method, however, the single gamma ray is generated, and hence the similar apparatus cannot be used. Since the atomic nucleuses building up the internal matter are light atomic nucleuses such as hydrogen, carbon and oxygen, the nuclear de-excitation gamma rays are limited to those having the energy equal to or higher than several MeV, and there is needed the imaging apparatus for the single gamma ray exhibiting the high energy such as this. A Compton camera is proposed as the only apparatus which fulfills this requirement, however, high-energy gamma-ray defection efficiency of the Compton camera is by far smaller than the detection efficiency of the gamma rays due to the positron annihilation, and it is difficult to measure the data sufficient for presuming the arrival depth of the particle beams and the energy impartation distribution.
Herein, the monitoring of the energy impartation distribution represents a technique of directly measuring both of “internal matter density” and “ion energy”. In the self-radiation method defined as the conventional method, the positron annihilation beam has only the intensity of the annihilation beam as a physical quantity for measurement because of the single energy (511 keV). Furthermore, both of the energy impartation and the annihilation beam intensity are proportional to the “internal matter density” and strongly depend on the “ion energy”. To take these points into consideration, the positron annihilation method requires the assumption of the “ion energy” within the body in order to monitor the energy impartation distribution, and has such a problem that it is impossible to monitor the energy impartation distribution directly.
A conventional particle radiation monitoring method had problems which follow and could not monitor information on behaviors of the particle beams.